Combustion of carbonaceous materials, such as coal, oil, natural gas and biomass is the dominant source of energy in today's industrial society. The primary products of combustion are heat, gases and ash. Heat generated by combustion is transferred to a working fluid, such as steam (making the system a "boiler"), which is then transported to a location where it is used to power turbines to produce electricity, drive chemical processes or provide a source of heat. Combustion is also used to incinerate solid municipal wastes. In this case, the primary product is the destruction of the waste, although some "waste-to-energy" systems make practical use of the heat generated by incineration. Combustion gases from boilers and incinerators are injected into the atmosphere after recovering as much heat as possible.
A typical boiler collects heat from both the combustion or furnace section and from the exhaust gas stream. Heat transfer in the furnace is primarily by absorption of the heat by water-cooled walls or tubing.
Combustion furnace designers and operators desire to monitor and control the operation of a boiler so that the performance of the boiler can be optimized and the efficiency of the boiler can be maximized, resulting in more efficient and cost-effective use of resources and less unwanted emissions. In utility boilers, the fraction of heat recovered is maximized when a particular temperature distribution is maintained within the boiler and its downstream recovery apparatus. When combustion temperatures or heat transfer temperatures deviate from this range, more heat is lost up the stack. This occurs, for example, when soot or slag builds up on the heat exchange surfaces of the combustion chamber thereby reducing the efficient transfer of heat to the boiler.
Incinerators for waste to energy production or for waste destruction must maintain minimum combustion temperatures in order to reduce the risk of emission of significant quantities of toxic hydrocarbons and/or chlorinated compounds. Exhaust gas temperatures are generally not monitored in these facilities, therefore procedures for assuring that these temperature requirements are met require use of excessive, and thus wasteful auxiliary fuels.
Certain pollution control systems for boilers or incinerators use a chemical process in the post-combustion zone to reduce the concentration of harmful pollutants. These systems inject urea, ammonia, or other compounds that react chemically with the harmful pollutants in the gas stream, rendering them benign. The reaction occurs within an optimum temperature range. Should these reactions occur at temperatures outside of the optimum range, the pollution reduction could be inadequate and other harmful compounds could be produced.
One of the parameters used to measure and control the efficiency of a boiler is the temperature of the gas exiting the combustion chamber. For many commercial boilers, it is desirable that the exit gas temperature be between about 1000.degree. K. to 1800.degree. K. When the temperature falls below this range, the combustion conditions can be changed to increase the temperature. When the temperature rises above this range, the heat transfer surfaces can be cleaned to improve heat transfer to the boiler. For example, an auxiliary heater is often used to control the temperature of combustion in solid waste incinerators. It is desirable to fire the auxiliary heaters only when necessary and only to the extent required to keep the combustion temperature within the desired range for maximum efficiency.
Attempts at providing reliable and accurate systems for monitoring exit gas temperatures have met with only limited success. Suction pyrometers, also known as high-velocity thermocouple probes, are generally used for this purpose. These devices are essentially thermocouples shielded by water-cooled tubular housings through which the hot exhaust gas is drawn. These devices are difficult to use and are not accurate unless the thermocouple junction is well shielded from the colder furnace walls. The thermocouples cannot withstand continuous exposure to the hot gases, and generally succumb to erosion and breakdown. Another drawback is that these devices only provide a single point measurement, so that several devices must be used to obtain an average gas temperature.
Acoustic pyrometers have also been used. Acoustic pyrometers are based on the premise that the change in the temperature of the gas can be related to the change in the speed of sound. These devices take a measurement across a line of sight to compute an average temperature. Acoustic temperature measurement assumes that the gas molecular weight is fairly constant, however, in practice the amount of moisture and the hydrogen content in the fuel can vary significantly, which renders sonic measurements less accurate. Another drawback is that the acoustic horns used in these devices are subjected to extremely high temperatures and soot and ash deposits which change their sound characteristics. For accurate temperature mapping, multiple horns and detectors are required. Sonic measurement is costly and complex, and requires time consuming signal analysis.
Infrared optical pyrometers have also been used to monitor exit gas temperatures. These pyrometers measure infrared radiation in the boiler exit chamber. However, they cannot distinguish between infrared radiation emitted by the gas and that radiating from the cooler furnace walls, thus, optical infrared pyrometers are not sufficiently accurate for use in industrial monitoring and control systems.
It is an object of the present invention to provide a method and apparatus which exploits an optical temperature monitoring device which accurately measures the temperature of exit gas, which can distinguish between the temperature of the gas and that of the walls, and which can be used to improve the control of a boiler, furnace or incinerator by regulating various combustion, heat transfer, pollution control and/or other chemical process parameters.